Field of the Invention
The present invention relates to an assembly for a semiconductor wafer manufacturing apparatus, more specially, to an inductively coupled coil and an inductively coupled plasma device using the same.
Description of the Related Art
Currently, with high development of electronic technology, there is stricter demand on integration level of IC, thus, IC enterprises have to improve manufacturing process of semiconductor wafers continuously. Plasma apparatuses or devices are widely used in manufacturing process of IC (Integrated Circuit) or MEMS (Micro-Electro-Mechanical System) devices. ICP (Inductively coupled plasma device) is widely used in etch process etc. Plasma is generated by RF gas ionization under low pressure. The plasma contains a large number of active particles such as electrons, ions, atoms, excited molecules and free radical etc. There occur chemical and physical reactions between these active particles and a material surface to be etched, to produce volatile resultants and change nature of the material surface.
An inductively coupled plasma device as shown in FIG. 1 is most commonly used in the current semiconductor etching apparatuses. During manufacturing process of semiconductors, process gas entering into a reaction chamber 3 through a gas inlet 2 on top of a dielectric window 1 is ionized into plasma by an inductively coupled coil 4 provided above, and the generated plasma etches material of the surface of a wafer 5. A molecular pump in the system pumps out discharging gas from the reaction chamber 3 through an outlet 6. During this process, the RF power from the inductively coupled coil 4 generates the plasma in the reaction chamber 3 by ionizing the gas. Presently, radio frequency of 13.56 MHz is applied to the inductively coupled coil 4 so that there is RF current in the inductively coupled coil 4, thus generating variable magnetic field. According to Faraday law, the variable magnetic field will induce electric field so that the reaction gas in the reaction chamber 3 is ionized into plasma. The excited plasma interacts with a workpiece in the chamber, etches the workpiece or deposits material on the workpiece. The workpiece is normally a wafer with circular plane. Due to the gas pumping manner or the asymmetry of the reaction chamber, asymmetry of particle density, temperature and magnetic flux in the reaction chamber would occur. Asymmetry of gas flow results in asymmetry of plasma conductivity, and the asymmetry of plasma conductivity may lead to non-uniformity of deposited power, which may lead to non-uniformity of ionization due to electron colliding. With size of the wafer 5 increasing, the volume of the reaction chamber 3 increases correspondingly. And the non-uniformity of plasma density from the center to periphery becomes more serious. Therefore, there is a problem in non-uniformity of etching rate for most of conventional etching apparatuses, which brings negative influence on the semiconductor manufacturing process.
To obtain relatively uniform etching rate on material surface to be etched, there is a need for achieving relatively uniform distribution of plasma density above the wafer 5 in the reaction chamber 3 so that there is a relatively uniform distribution of plasma above the wafer 5, thus improving quality of etching.
In FIG. 2, a commonly used structure of an inductively coupled coil 4 is shown, which is a planar-and-spiral configuration. However, the plasma excited by this configuration is very non-uniform. Because the electromagnetic field induced by this configuration at the center of the reaction chamber is relatively strong, the plasma density at the center thereof is relatively high, and low plasma density at the periphery can only be remedied via diffusion. This will lead to strict dependency on the gas pressure which only brings most favorable performance in the range of 1-10 m Torr. This may lead to a small adjustable window to the process and great limitations to the semiconductor manufacturing process. When the plasma density distributes uniformly, the etched depth on the wafer or the thickness of the material deposited on the wafer is non-uniform, thus the yield of the devices is also decreased. Especially, when the diameter of the wafer increases from 100 mm to 300 mm, the volume of the reaction chamber increases correspondingly, and it is impossible to obtain uniform plasma density via diffusion.
In addition, when the diameter of the wafer reaches 300 mm, the size of the inductively coupled coil should be correspondingly increased, and the plasma chamber for processing the wafer should also be increased. Therefore, the thickness of the dielectric window 1 is needed to increase accordingly. Otherwise, the dielectric window 1 may not endure the pressure difference between the atmospheric pressure outside the chamber and the vacuum inside the chamber. The dielectric window 1 having such an thickness will lead to decrease of energy coupling efficiency, since there is no enough flux density to excite plasma after the RF field penetrating through the thick dielectric window 1. And the problem with the conventional spiral inductively coupled coil is the generation of non-uniform plasma density. The length of the conventional inductively coupled coil may increase remarkably with the increase of wafer radius, even beyond ⅛ of a RF source wavelength. Thus, the transmission line effect of the inductively coupled coil is obvious, and there exists remarkable variations of current and voltage in the inductively coupled coil, which results in remarkable variation of the magnetic flux density in the plasma and non-uniformity of workpiece processing. On the other hand, when the size of the inductively coupled coil increases, the corresponding inductance will also increase, thus the voltage of both ends of the inductively coupled coil will increase correspondingly. The increased voltage will result in capacitive coupling between the inductively coupled coil and the plasma, and the capacitive coupling increases the kinetic energy of the ion, so that it is hard to control the process precisely. In addition, it will increase micro-loading effect of the wafer and decrease yield of the devices. Further, ions with relatively high kinetic energy collide with the inner walls of the plasma chamber, which will result in particle contamination. And increased inductance will also lead to unstable impedance match and low coupling efficiency. Correspondingly, the non-uniformity of radial plasma density will increase.